A SAW device typically comprises two sets of interdigital electrodes, formed at opposite ends on a surface of a piezoelectric crystal or other suitable substrate. Conventional photolithographic and etching techniques are used to produce the electrode pattern. When a radio frequency (RF) voltage is applied to the electrode pair on one end of the device, a Rayleigh surface acoustic wave is generated that propagates over the substrate, toward the other pair of electrodes, on which is developed a corresponding RF voltage. Since the surface acoustic wave propagates by mechanical deformation of the crystal and structure in the region between the pairs of electrodes, any material applied as a coating on that region of the substrate can affect certain characteristic parameters of the surface acoustic wave, such as its amplitude, velocity, and phase. Furthermore, when the material is exposed to a chemical substance, e.g., a vapor, these parameters change as the chemical substance is absorbed by the material coating the substrate. The change in mass of the coating resulting from its absorption of the chemical substance causes a proportional change in the phase of the RF signal output from the electrodes that are excited by the surface acoustic wave. Similarly, the amplitudes of the surface acoustic wave and of the output signal change in proportion to the amount of the chemical substance that is absorbed, although the sensitivity of the device to such change with respect to amplitude of the output signal is much less than with respect to frequency.
By selecting the material coating applied to the substrate, which is typically an organic polymer, for its characteristic solubility in a desired chemical substance, the SAW device can be used to detect that specific chemical substance. Since other operating parameters, such as ambient temperature, may effect the signal output from the chemical sensor SAW device, it is preferable to compare a signal output from a reference SAW device formed on the same substrate (but coated with a material that does not absorb chemical substances). Since both the reference and chemical sensor SAW devices are exposed to the same environment, any difference between their output signals is indicative of absorption of a chemical substance by the one SAW device.
SAW devices used for detecting chemical substances in the manner described above are known in the art, as exemplified by the disclosure of U.S. Pat. No. 4,312,228. Such devices are commercially available from a company that pioneered their early development, Microsensors Systems, Inc., Fairfax, Va. To construct a chemical detector that can sense more than one substance using presently available SAW devices, a plurality of SAW sensors are normally employed, each sensor including a different organic polymer coating selected for its solubility in a different chemical substance or group of chemical substances. Detection of a specific chemical substance is effected by analyzing the signals produced by the plurality of different SAW devices when exposed to an unknown substance, matching the pattern of responses from all the sensors to a set of known patterns. Prior art SAW chemical sensors have generally relied only on the change in the frequency of the output signal, when exposed to an unknown substance, and not on changes in amplitude. As the number of different chemical substances of interest increases, so does the required number of different SAW sensors needed to identify a particular substance from the variety of substances that might be present. For certain applications, the cost and complexity of such a chemical substance detector may be excessive.
In consideration of the preceding problem, it is an object of the present invention to provide a SAW sensor for detecting a plurality of different chemical substances. A further object is to provide a SAW sensor in which both a frequency change and an amplitude change in the output signal, resulting from exposure of the sensor to a chemical substance is used in detecting and identifying the substance. Yet a further object is to provide a SAW sensor in which a range of surface acoustic wave frequencies are generated to detect different chemical substances, where the sensitivity of the material coatings on the sensor to different chemical substances varies and the range of frequencies applied to a specific material coating is selected to compensate for the variation in sensitivity. Still a further object is to minimize errors in determining changes in phase in the SAW sensor when it is exposed to a chemical substance, where such errors result from phase ambiguity. These and other objects and advantages of the invention will be apparent from the attached drawings and the Description of the Preferred Embodiments that follow.